Dependences of wear resistance and strength on carbon concentration and microstructure distributions for a 18Cr2Ni4W alloy steel with deep carburized layers: Finite element modeling and experiments

Abstract

In the present study, the effects of microstructure, carbon concentration, and hardness distributions on wear resistance and tensile properties of a 18Cr2Ni4W alloy steel with deep carburized layers were investigated through experiments in conjunction with finite element simulations. After incorporating the thermophysical database of 18Cr2Ni4W in DEFORM finite element software, it was used to simulate carburizing at temperatures of 930 °C, 950 °C, 970 °C, respectively, and the subsequent heat treatment processes to obtain hardened layers with a 4.5 mm depth. The distributions of carbon concentrations, martensite, and retained austenite fractions after carburizing heat treatment were simulated and validated against experimental data. The results reveal that the surface carbon concentration and hardness increased with the rise of carburizing temperature. The measured hardness distributions in experiments are consistent with the DEFORM simulations, indicating the validity and accuracy of the finite element model. Tensile tests of the carburized surface indicates that the tensile strengths of the samples obtained from different carburizing temperatures all exceed 2000 MPa. Ductility decreases significantly with the increasing carburizing temperature. The sliding wear tests show that the sample carburized at 970 °C has the smallest wear rate (1.33 × 10⁻⁸ mm³ N⁻¹ mm⁻¹) as compared to the 930 °C and 950 °C cases, in which Si₃N₄ counterbodies were applied. The wear resistance of the sample carburized at 950 °C is merely better than that carburized at 930 °C. The primary wear mechanisms are found to be adhesive oxidation wear accompanied by abrasive wear. The morphology and element distribution tests of the Si₃N₄ counterface pin after the friction and wear experiments indicate the materials transfer from the carburized 18Cr2Ni4W alloy surface to the counterbody. The most obvious aggregation of element oxygen was found on the counterface that contacts the 930 °C carburized sample, implying the most severe adhesive oxidation wear. Finite element simulation of temperature fields during friction and wear show that the maximum temperature at the contact point of the sample carburized at 970 °C was 50 °C lower than that of the sample carburized at 930 °C, so that the oxidation wear of the sample carburized at 970 °C was significantly alleviated, which accord with the experimental results. Therefore, the increase of the volume fraction of carbides, along with the reduction of temperature during friction, enhanced the wear resistance of the carburized layer. This study elucidates the significant effect of carbon concentration and microstructure on the wear resistance and strength of 18Cr2Ni4W alloy steel, providing scientific guidance for optimizing carburizing heat treatment to improve surface wear resistance.